Reflector for circularly polarized waves



March 10, 1970 R. PIERROT 3,500,420

REFLECTOR FOR CIRCULARLY POLARIZED WAVES Filed Dec. 15, 1967 2 Sheets-Sheet 1 March 10, 1970 o-r 3,500,420

REFLECTOR FOR CIRCULARLY POLARIZED WAVES L L L Fig.6

United States Patent Int. (:1. H01 19/00 US. Cl. 343-756 4 Claims ABSTRACT OF THE DISCLOSURE A reflector element for circularly polarized waves, comprising a conductor, the length of which is equal to where )t is the Operating wavelength and N is an integer. The conductor comprises a linear intermediate section parallel to the Wave propagation direction, of length equal to h A V 1 where n is an integer smaller than N, and two linear end sections of equal lengths, each section being perpendicular to the other two.

The present invention relates to semi-transparent electromagnetic systems.

More particularly, it relates to elements which are resonant when they are excited by a wave having a circular polarization of a given direction, and are not resonant with a wave having a circular polarization of the opposite direction.

The invention also relates to reflecting systems comprising such reflectors, such as, for example, systems of the so-called Cassegrain type, aplanatic systems, reflecting systems without source shadow, and more generally systems based on the use of at least one semi-transparent reflector.

According to the invention there is provided a reflector element for waves propagating along a given path with a circular polarization, the electric field vector of which rotates in a predetermined direction, said element being conductive and compromising an intermediate linear section, and two linear end sections, each section being perpendicular to both others, said intermediate section being parallel to said path, the total developed length of the element being equal to where N is an integer and A is the operating wavelength, the length of the intermediate section being equal to A A fi e where n is an integer smaller than N, and the two end sections having equal lengths.

For a better understanding of the invention and to show how the same may be carried into effect, reference will be made to the drawings accompanying the following description and in which:

FIG. 1 shows a resonant element according to the invention:

FIGS. 2, 3, 4 and 5 are explanatory diagrams;

FIG. 6 is a basic diagram of a reflector system according to the invention; and

'ice

FIG. 7 is an example of an application of a reflector according to the invention.

As is known, a Wave, linearly polarized parallel to a conducting wire, induces a current in this wire. The wire excited in this way starts to resonate if its length is a multiple of M2, and behaves like a reflector relative to the wave. On the other hand, if the wire is perpendicular to the direction of polarization of the wave, the same is propagated without change, at least theoretically.

As is also known, 'a circularly polarized wave may be regarded as the resultant of two component waves polarized in two mutually perpendicular directions and phase shifted by with respect to each other.

The element according to the invention, shown in FIG. 1, comprises an intermediate section BC with a length d, parallel to the direction W of propagation of the wave and two end sections AB and CD having the same length, perpendicular to BC and perpendicular to each other. One of the end sections is excited by the component of the electric field parallel thereto and the other by the other component.

Let E and E designate, as shown in FIG. 2, the vertical and horizontal components of the electric field of a right-hand circularly polarized wave which is propagated in the direction W parallel to BC, E lagging by 90 with respect to E The field E induces in the section AB a current i which, at a given moment, has the direction indicated in FIG. 1, whilst the field E which has no effect on AB (neither of these fields has of course any effect on the section BC) induces the same current i (in phase or with the opposite phase to the first) in the section CD, provided that the length of the centre BC is equal to h/ 4 to Within rely i.e. to

The total length L of the element A B- C D must of course to a multiple N of M2 in order to be able to enter into resonance with the wavelength A, i.e.,

the length l of each end section being equal to 7\ l= [2(Nn) 1] If N is odd, there will be an even number of nodes: the currents in the end sections must be in phase for the element to resonate.

FIG. 4 shows the case for N=l.

If, on the other hand, N is even, there will be resonance, if the currents are opposite phase.

FIG. 5 shows the case for N =2.

On the other hand, according to whether n is odd or even (or zero), the current induced in the arm CD has the direction shown in FIG. 1, or the opposite direction for a right-hand circular polarization.

Hence, for a right-hand circular polarization and for an element such as shown in FIG. 1, there will be total reflection of the wave if N is odd and n is even or zero, or if N is even and n is odd.

Naturally, the results will be reversed, if either the direction of polarization is reversed or that of the section CD.

Since also the length l of AB or CD must be less than M 2, it follows that either N=n+1, or N=n+2, i.e.

The choice of It depends substantially on the wavelength: the dimensions of the element must be both sufficiently small for the reflector according to the invention, shown in FIG. 6 and formed by a panel of elements of FIG. 1, not to be of prohibitive dimensions, and sufliciently large to permit easy manufacture at acceptable cost.

FIG. 6 shows in perspective a semi-transparent reflector according to the invention formed by a panel of left-hand or right-hand elements in which all elements of the panel are identical. The elements are held in position, for example, by a resinous foam M.

The spacings rs, and e between two consecutive ele ments, connected, respectively, to the first and last sections, must be small compared with the wavelength. Since they are, on the other hand, limited towards the smaller size by the length l of the end sections, it is preferably to give these elements minimum dimensions, i.e.,

and

which has also the further advantage of an operating band which is wider than that corresponding to maximum dimensions, whilst the ratio between the variation of one'dimensions of the element and the variation of the wavelength is smaller.

The wire diameter will be small compared with the wavelength (of the order of The panel shown in FIG. 6 has the form of a parallelepiped. In practice, it can have any shape, provided that the radii of curvature are large compared with the dimensions of one single element.

FIG. 7 shows the diagram of an antenna with two reflectors operating with circular polarization according to the invention.

The primary source S radiates circularly polarized waves, for example, right hand polarized waves.

The front reflector R is formed by a semi-transparent panel according to the invention, transparent for waves with left-hand circular polarization and reflecting with regard to the waves with right-hand circular polarization.

The waves reflected by R without change in the direction of polarization, are then reflected by the metal panel R with reversal of the direction of polarization, and pass then through the first reflector.

Naturally, the invention is not limited to the em'bodiments shown and described above, which are given merely by Way of example.

More particularly, the use of the element and of the reflector is not limited to the application thereof given above. Thus, for example, the reflector according to the invention may be used equally well in systems with two reflectors, for example, of the type without source shadow and/or aplanatic reflectors described in the U.S. patent application, Ser. No. 536,880, filed Mar. 23, 1966, entitled Plural Reflector Antenna With Polarization Rotation to Minimize Feed Shadow, now Patent No. 3,430,246.

What is claimed is:

1. A reflector element for waves propagating along a given path with a circular polarization, the electric field vector of which rotates in a predetermined direction, said element being conductive and comprising an intermediate linear section, and two linear end sections, each section being perpendicular to both others, said intermediate section being parallel to said path, the total developed length of the element being equal to where N is an integer and x is the operating wavelength, the length of the intermediate section being equal to A A We where n is an integer smaller than N, and the two end sections having equal lengths.

2. A reflector element according to claim 1 wherein said conductor is a wire.

3. A semi-transparent reflector for circularly polarized waves, comprising an array of identical reflector elements according to claim 1.

4. A reflector antenna system for circularly polarized waves comprising a radiating source for radiating, along a given path, waves with a predetermined type of circular polarization, a first reflector according to claim 2, positioned on said path, the elements of said reflector being reflective for said predetermined polarization, and a second reflector for reversing the sense of rotation of the field vector of circularly polarized waves incident thereon.

References Cited UNITED STATES PATENTS 2/1956 Cochrane 343-756 1/1959 Mariner et al 343-756 U.S. Cl. X.R. 

